1. Field of the Invention
The present invention relates to a carbon-monoxide removing (removal) catalyst for removing mainly carbon monoxide from a gas containing hydrogen (H2) gas as the major component thereof and containing also a small amount of carbon monoxide (CO) gas, such as a reformed gas obtained by reforming (steam reforming, partial oxidation reforming, etc.) a hydrocarbon such as a natural gas, naphtha, kerosene or the like and an alcohol such as methanol. The invention relates also to a method of removing carbon monoxide therefrom.
2. Description of the Prior Art
Conventionally, with a fuel reforming apparatus for manufacturing reformed gas (gas containing 40 volume % or more (dry base) of hydrogen) with using fossil fuel such as natural gas as raw material, the raw material was desulfurized and steam-reformed through a desulfurizer and a steam reformer disposed one after another, thereby to obtain the reformed gas containing hydrogen as the major component thereof and carbon monoxide, carbon dioxide (CO2), water (H2O), etc. Further, with a fuel reforming apparatus using an alcohol such as methane as the raw material, the apparatus includes a methanol reformer incorporating a methanol reforming catalyst, thereby to obtain, from methanol, a reformed gas containing hydrogen as the major component thereof and carbon monoxide, carbon dioxide, water, etc.
Here, with a fuel reforming apparatus for making a reformed gas for use in a phosphoric acid fuel cell, it is known that the electrode catalyst of the fuel cell is poisoned by the presence of carbon monoxide. Therefore, in order to prevent poisoning of the electrode catalyst, the gas containing hydrogen as the major component thereof was introduced to a carbon-monoxide shift converter for converting carbon monoxide into carbon dioxide (CO2) through a carbon monoxide shift converting reaction, thereby to obtain a reformed gas with the carbon monoxide concentration in the gas being lower than a predetermined value (e.g. 0.5%).
However, in the case of a fuel reforming apparatus for producing a reformed gas for use in a polymer electrolyte fuel cell, since this polymer electrolyte fuel cell operates at a low temperature of about 80° C., its electrode catalyst will be poisoned even if just a trace amount of carbon monoxide is present. Therefore, it is necessary to further reduce carbon monoxide to be contained in the reformed gas. So, on the downstream of the carbon monoxide shift converter, there was provided a carbon monoxide remover incorporating a carbon monoxide removing catalyst for removing carbon monoxide. With this, the reformed gas treated by the carbon monoxide shift converter was introduced, with addition thereto of an oxidizer such as air, to the carbon monoxide remover, so that carbon monoxide was oxidized into carbon dioxide in the presence of this carbon monoxide removing catalyst, whereby a reformed gas with reduced carbon monoxide concentration lower than a predetermined concentration (e.g. 100 ppm or lower) was obtained.
As this type of carbon monoxide removing catalyst, there is employed a precious metal catalyst comprising ruthenium (Ru), rhodium (Rh), platinum (Pt), palladium (Pd) or the like supported on a support made of e.g. alumina. And, conventionally, such catalyst was directly put for use in the elimination of carbon monoxide, without effecting any activating treatment on the catalyst. Or, there was proposed an activating method in which the carbon monoxide removing catalyst is subjected to a pre-treatment in a gas atmosphere containing hydrogen as the major component thereof (50 mol % or more) and then the catalyst is put to use without being exposed to air (see Japanese Patent Application “Kokai” No.: Hei. 10-29802). This may be because exposure to air is believed to lead to reduction in the catalyst activity.
However, in order to remove carbon monoxide from the above-described reformed gas to achieve its concentration of 10 ppm or less by using the conventional carbon monoxide removing catalyst, it was necessary to add an excessive amount of oxidizer (oxygen) thereto. Moreover, when the carbon monoxide removing catalyst is to be used at a low temperature (e.g. near 100° C.), its catalyst activity is low, so that carbon monoxide could not be removed effectively. Accordingly, in order to remove a greater amount of carbon monoxide, it was necessary to use the carbon monoxide removing catalyst at a high temperature range (near about 200° C.) so as to enhance its activity.
When carbon monoxide is to be removed from the above-described mixture gas containing hydrogen and carbon monoxide, it is known that the carbon monoxide removing catalyst employed would provide not only the useful effect of removing carbon monoxide, but also side reactions which consume the hydrogen contained in the mixture gas to produce carbon monoxide, methane, and water (respectively referred to as a reverse shift reaction of carbon dioxide, a methanation reaction of carbon dioxide, and combustion reaction of hydrogen). Especially, these side reactions are apt to occur when the temperature of the carbon monoxide removing catalyst is high (e.g. 200° C. or higher).
Therefore, if the carbon monoxide removing catalyst is used at a high temperature range in order to remove a greater amount of carbon monoxide, there occurs the problem of the above-described methanation reaction being very much promoted. This is problematic not only in that the hydrogen needed by the fuel cell is consumed inadvertently in the methanation reaction, but also in that the temperature will be further elevated due to the reaction heat from the methanation reaction. Moreover, there is still another problem of the carbon monoxide removing catalyst being poisoned with iron, thus resulting in performance degradation.
In this regard, the following mechanism is believed to be responsible for the poisoning of the carbon monoxide removing catalyst with iron. First, when a high-temperature reaction gas containing hydrogen and carbon monoxide is introduced into the carbon monoxide remover, bonding occurs between the carbon monoxide and iron contained in the stainless steel forming a reaction tube of the carbon monoxide remover, thereby to produce a compound similar in structure to iron carbonyl (Fe(CO)5). As this iron carbonyl moves together with the mixture gas to adhere to the catalyst portion of the carbon monoxide remover, this carbon monoxide removing catalyst will be poisoned. One method to avoid this poisoning of the carbon monoxide removing catalyst with iron, there is known a method for rendering the temperature of the reaction gas to be introduced to be lower than 100° C. so as to prevent production of the iron carbonyl inside the reaction tube. As described above, such method for protecting the carbon monoxide removing catalyst against iron poisoning is also required.
Moreover, if a large amount of water is contained in the reaction gas to be introduced into the carbon monoxide remover, the water will aggregate and form dew within the pipe or carbon monoxide remover if the temperature of the reaction gas introduced to an inlet of the carbon monoxide remover is reduced to 100° C. or lower. The dew formation can result in random variations in the cross sectional area and the volume of the reaction gas passage within the carbon monoxide remover, which results, in turn, in random variation of the flow rate of the reaction gas being supplied into the carbon monoxide remover and/or in wetting of the carbon monoxide removing catalyst housed in the carbon monoxide remover with the aggregated water, leading to reduction in its activity.
The present invention has been made in view of the above-described drawbacks and its object is to provide a method of activating a carbon monoxide removing catalyst for activating the carbon monoxide removing catalyst for removing, mainly through its oxidation, carbon monoxide present in a mixture gas containing hydrogen and the carbon monoxide by causing the catalyst to contact an inactive gas or a hydrogen-containing inactive gas consisting of less than 50 volume % of hydrogen gas and the remaining volume of inactive gas.